Flickering and funky cameracontrolls in perspective-view
Hello and thank you very much for your comments!

The problem was indeed "Clip: Start". As soon as i changed the value all problems were gone. Visuals and controlls restored to normal.
But this must have been a bug, since i could not reproduce the problem by setting the value back to 0.001.

I hope some people can use this workaround in case they run into the same glitch.

Thank you for your Help!